The present invention relates generally to optical instruments, and more specifically the invention pertains to a telescope baffle system for eliminating the off-axis stray radiation which, in current baffle designs reaches the image plane on optical telescope radiation of images captured in an optical telescope.
A telescope is an instrument used to collect radiation for the study of distant objects. A telescope is commonly regarded as an optical instrument that images visible light by means of lenses, mirrors, or both, to augment the eye with respect to resolution and light-gathering power. Instruments designed to collect invisible radiation, as gamma rays, x-rays, cosmic rays, and other atomic particles, as well as ultraviolet and infrared light and radio waves, are also called telescopes.
Although the technology of making lenses for eyeglasses was begun late in the 13th century, it was not until the beginning of the 17th century that telescopes came into use. When lenses are used to form images, optical instruments are called refractors. Sophisticated examples of the refracting telescope, made famous by Galileo, include astronomical and terrestrial telescopes with objectives (the first image-forming lens) up to 40 in (1.016 m) in diameter. Binoculars, opera glasses, gunsights, theodolites, periscopes, range finders, and cameras in great variety are other examples.
The images captured by these various systems are all subject to contamination by undesired off-axis radiation.
The task of eliminating off-axis radiation from optical instruments is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,895,428 issued to Nelso et al; PA1 U.S. Pat. No. 4,542,963 issued to Linlor; PA1 U.S. Pat. No. 3,488,103 issued to Wirick; PA1 U.S. Pat. No. 4,529,273 issued to Cronin-Golomb; PA1 U.S. Pat. No. 3,445,659 issued to Guimento et al; PA1 U.S. Pat. No. 3,699,471 issued to Mulready et al; PA1 U.S. Pat. No. 4,217,026 issued to Radovich; PA1 U.S. Pat. No. 4,431,917 issued to Gibbons and PA1 U.S. Pat. No. 4,507,551 issued to Howard et al.
The Linlor patent discloses an optical system with reflective baffles. In Linlor, reflective baffles extend circumferentially around the tube of a telescope. The Linlor baffles are reflective on both their forward (input) sides as well as their rearward sides.
In addition to the Linlor system, others have given their attention to the use of reflective baffles in optical systems. For example, Davis, U.S. Pat. No. 3,488,103, issued Jan. 6, 1970 discloses a reflecting baffle having a concave, elliptical surface facing the direction off-axis rays enter an optical system. However, in order for the system there disclosed to exclude off-axis radiation sufficiently, the field of view of the optical system must be substantially reduced. Radovich, U.S. Pat. No. 4,217,026, issued Aug. 12, 1980, discloses an optical system incorporating a plurality of baffles that are each also concave with respect to the incident radiation to be reflected. This system also results in substantial reduction of the field of view, and it is only partially effective for rejecting off-axis rays. A similar system is described by Rock et al, "Use of Reflective baffles for Control of Aperture Heat Loads and Stray Radiation," Optical Systems Engineering, Proceedings of SPIE, Vol. 330, pp. 60-65, January 1982, in which radiation absorbing surfaces are placed in the vicinity of the first baffle, in order to prevent skew rays from reaching the image plane. Such a system is also disclosed by Bremer, "Baffle Design for Earth Radiation Rejection in the Cryogenic Limb Scanning Interferometer/Radiometer," Optical Engineering, Vol. 22, No. 1, pp. 166-171, January-February 1983.
The purpose of the baffle section of an optical sensor is to prevent unwanted off-axis radiation from reaching the telescope and then being scattered onto the detector. In the past this has been accomplished by using a baffle tube with internal annular rings which are coated with some type of black absorbing material, usually black paint. The black coating does not absorb 100 percent of the off axis radiation, which results in a fraction of the unwanted radiation being scattered to the optical surfaces of the telescope. The telescope optical surfaces in turn scatter a portion of the radiation into the field of view where it is then focused onto the detectors of the optical sensor. The amount of unwanted off-axis radiation which eventually reaches the detectors depends on the effectiveness of the baffle and the scatter properties of the telescope optical surfaces. In addition, the black coating is usually not equally effective at all wavelengths and poor off-axis rejection can occur at wavelengths where the coating is a poor absorber. An infrared sensor would usually require the baffle to be cooled to prevent thermal emission from the baffle. The black coating absorbs off-axis thermal radiation which tends to increase the baffle temperature and increases the heat load on the sensor which increases the refrigeration required to keep the baffle cold.
The optical baffle systems of the past seem to be divided into two categories: baffles which are entirely absorptive; and baffles which are entirely reflective on both sides. Both systems have advantages but present the designer of optical systems with a faulty dilemma: the baffles need not be entirely absorptive nor entirely reflective, but can adopt the advantages of both designs. Additionally, the use of the phase conjugate mirror of the above-cited Cronin-Golomb reference allows the telescope baffles to present a retro-reflective array which reflects back off-axis rays at their angles of incidence. However, the use of true phase congugate mirrors is impractical.